What exactly is the non-metallic compensator used in the fan system? What are the advantages over metal bellows?
Don't look at the parameters in a hurry. Let's find out one thing first: the pipe at the outlet of the fan has high temperature, big vibration, and the flue is often twisted. If you take it hard, the welds will tear, the pipes will displace, or even the fan casing will be pulled and deformed-I have seen this kind of accident on the scene more than once. Non-metallic compensators (that is, we often call non-metallic expansion joints) do this specifically: absorb thermal displacement, reduce vibration and noise.
Compared with metal bellows, what are the advantages of non-metal? Metal bellows are afraid of corrosion and fatigue, especially when there is sulfur and acid and alkali in the flue gas, and stainless steel will perforate in a few months. The main body of non-metallic expansion joint (fabric fiber expansion joint) is silicone cloth + glass fiber + fluororubber, which is resistant to acid and alkali and high temperature (350℃ for a long time and 450℃ for a short time), and it is a flexible structure itself, which can absorb axial, transverse and angular displacements at the same time-metal bellows have to use a double hinge structure to do this, and the cost is doubled. In addition, the non-metallic compensator has a small thrust on the pipe, and does not need to be equipped with a large number of tie rods or pressure balancing structures like metal bellows.
At the outlet of the induced draft fan of the power plant, the round tube of DN2800 has a temperature of 280℃. If a non-metallic expansion joint (fabric fiber expansion joint) is used, the cost of a single set is about 30% lower than that of a metal bellows. The error can be trimmed during installation, and the installation period can be saved half a day.
In the second construction examination and actual projects, how can the four most headache parameters of selection-temperature, pressure, displacement and media corrosivity be matched?
The second construction textbook talks about the selection of expansion joints, and the four parameters are turned over and over. But when actually choosing, the easiest thing to roll over is the temperature. For example, the flue temperature at the outlet of the fan is nominal to 300℃, but you have to consider the impact at the moment of opening the furnace-it may rush to 400℃. If you use regular silicone cloth for your non-metallic expansion joint, it's screwed, bulging and carbonizing in minutes. At this time, you have to choose the high-temperature resistant type, such as this station'sNon-metallic expansion joint (fabric fiber expansion joint)Its composite layer is added with high-temperature resistant fluororubber and glass fiber, which can withstand 450℃ for a short time.
For pressure, the fan system is mostly slightly positive pressure or negative pressure (within ± 0.05MPa). Don't take the standard of medium and high pressure bellows. If the pressure exceeds 0.1MPa, the non-metallic expansion joint must be reinforced skeleton or used insteadrubber compensatorOrRubber PTFE compensator— — The latter has stronger corrosion resistance and is suitable for wet flue gas after desulfurization.
How to calculate the displacement amount? Don't just count thermal expansion. When the fan starts and stops, vibration displacement will occur, and the self-weight settlement of the flue will also bring displacement. Generally, it is recommended to leave a margin of 15% ~20% for the displacement. For example, calculate the axial displacement of 30mm, and select the type according to 35mm ~40mm. This site'sRectangular non-metallic expansion jointIt is particularly practical on rectangular flue, and has angular compensation ability at four corners, which is suitable for working conditions with narrow space and complicated displacement direction.
Media corrosiveness this pit is the most. Ordinary flue gas contains sulfur, and fluororubber among non-metallic materials is the most corrosion-resistant, followed by silicone rubber. If the flue gas contains hydrofluoric acid (such as the residue after wet desulfurization), fluororubber can't bear it, so you have to choosePTFE compensator-Resistant to almost all chemicals. In the second construction examination, the classification of media is often examined. Remember: PTFE or fluororubber is selected for acidic media; Select basic or neutral silica gel; Nitrile rubber is selected as oily medium.
Will the tie rod be disassembled or not during installation? How to adjust the nut? Combined with the pits often stepped on at the second construction site, make it clear directly.
This problem, I have to talk to the construction team for a long time every time I go to the project. The tie rod on the non-metallic expansion joint, is used to prevent excessive deformation of the compensator when transported and installed, not for you to fix permanently! After installation and pipeline connection is fixed, the tie rod must be removed or the nut loosened to the set position. And guess what? I have seen a construction site where the tie rod was not removed. As soon as the fan was started, the compensator was held tightly. As a result, two flange bolts broke, and there was almost an accident.
How to adjust the nut? Refer to Q&A on this site"How to adjust the tie rod nut of the expansion joint": First confirm the working displacement direction of the compensator according to the design drawings, then loosen the locking nut with a wrench, adjust the tie rod nut to the installation length reserved by the design, and finally lock the locking nut again. Remember a formula: the factory state is locked, pre-adjust when the installation is in place, and then unlock after the pipeline is connected-unlocking means loosening the nut, so that the tie rod no longer bears the preloading force, but the tie rod itself is not disassembled, and it can also play a limiting role in preventing the compensator from over-stretching.
The flange bolts are tightened too tight. The flange surface of non-metallic expansion joint is generally soft connection. If the bolt is tightened too tightly, the pressure plate will be deformed or even the fabric layer will be crushed. Standard torque: M16 bolt torque 80~100N·m, do not exceed 120N·m. How to judge? Leave a gap of 1~2mm between the edge of the pressure plate and the flange surface.
After two years of use, it has leaked and bulged, so don't rush to change it-first judge whether it is fatigue aging or medium corrosion, and then compare the fabric fiber expansion joint and rubber PTFE compensator of this station to see whether it should be repaired or replaced.
There is a large bulge on the surface of the non-metallic expansion joint at the outlet of the fan, and air leaks. He directly wanted to replace it with a new one, and I asked him to take it apart first to see where the bulge was. If the surface bulge is in the middle of the fabric layer, it is mostly peeled off between layers, which belongs to fatigue aging. Usually, the service life of non-metallic expansion joints is 3~5 years. If the operating temperature is always on the high side (exceeding the design temperature by 20%), the service life will be directly cut in half. This bulge, if only partial, can be patched and repaired-covered with silicone cloth of the same material and hot pressed, but only if the inner anti-corrosion layer is not broken.
If the inner anti-corrosion layer (fluororubber or tetrafluorine film) has been perforated, embrittled, and cracked, it is caused by media corrosion. The patch is useless at this time and must be replaced as a whole. Don't buy indiscriminately when replacing, compare the working conditions: medium corrosiveness strong selectionRubber PTFE compensator(tetrafluorine membrane + rubber composite); High temperature and weak corrosion selectionNon-metallic expansion joint (fabric fiber expansion joint)； If there are strong acid and alkali and vibration at the same time, it is recommended to usePTFE compensatorPlus stainless steel skeleton.
The sealing surface of the flange is uneven. Some sites have the compensator installed crooked, or the flange is deformed. In this case, you only need to remove and reinstall it, add a sealing gasket (graphite gasket or PTFE gasket), and don't need to change the compensator itself. Don't waste your money.
Two days ago, a friend who was doing the second construction project asked me: The outlet temperature of the fan is 450℃, and the flue is narrow. Do you choose non-metal or metal? My answer is...
His project is a cement kiln head grate cooler fan. The outlet flue space is only 200mm wide, with an axial displacement of 40mm and a lateral displacement of 15mm. If you choose metal bellows, the single-layer stainless steel will definitely be oxidized at 430℃, so you have to use Inconel alloy, and the cost will be directly doubled by 5 times. Moreover, the space is narrow, and the double hinge structure of the metal bellows can't be placed at all. So my answer is: use non-metallic expansion joints, but you must choose high-temperature resistant types.
- High-temperature non-metallic expansion joint (fabric fiber expansion joint): The short-term temperature resistance is 450℃, the long-term temperature resistance is 350℃, the price is about 800 yuan/m (calculated according to the diameter), the installation space only needs 150mm, it can absorb the comprehensive displacement of 40mm in the axial direction +15mm in the transverse direction, and the thrust on the pipeline is less than 500N.
- Metal Bellows (Inconel625): The short-term temperature resistance is 450℃, the long-term temperature is 400℃, the price is about 4500 yuan/m, the installation space is at least 250mm (because the double hinge needs to be added), the thrust on the pipeline is about 3000N, and an additional cooling sleeve is required.
The non-metallic expansion joint saves 80% space, reduces costs by 82%, and has a small thrust, so it does not need to reinforce the fan bracket. After hearing this, he placed an order directly from this siteNon-metallic expansion joint (fabric fiber expansion joint)Which is equipped with a high-temperature composite layer. It has been a year since the installation, and there is no air leak or bulge.
Therefore, don't think that non-metals can't work when you hear 450℃. The material technology of non-metallic expansion joints has made great progress in recent years. The temperature resistance limit of fluororubber has reached 500℃ (short time), and the cost is far lower than that of nickel-based alloys. If you encounter this kind of question in the second construction exam, remember:Second construction of non-metallic compensator for fanSelection formula-high temperature selection fabric compound, corrosion selection PTFE rubber, narrow space selection rectangular non-metal, small pressure difference is not afraid of soft connection.